This invention relates to the characterization of a flowing system such as a fluid or a mixture of fluids or a mixture of liquids with gases and/or solids by determining the rheological properties of the flowing system. More particularly, the invention relates to an improved rheometer having an adjustable inner cavity dimension so that properties of the flowing system are easily varied and the flowing system can be characterized without altering the volumetric flow rate of the flowing system.
Generally, rheometers operate by forcing the flowing system to be characterized through a die at a constant volumetric flow rate and determining specific properties of the flowing system such as the shear rate and shear stress of the flowing system at the wall of the die in order to determine the shear viscosity. These properties are characteristic of the flow and deformation behavior of the flowing system. The shear viscosity, in particular, is the most important rheological property.
The flow and deformation behavior are related to primary characteristics of the flowing system such as molecular weight, molecular weight distribution, extent of chain branching in polymer melts, degree of cure or conversion in a reacting system, and various microstructural distributions in multiphase systems. The microstructural distributions include the size distributions, locations and orientations of solid components in the flow, the concentration, shape, surface and size distributions of liquid droplets and the content and size distributions of gaseous components. Thus, characterization of the rheological behavior of a flowing system provides information on the structure of the system.
The rheological properties of the flowing system are characterized by employing material functions such as wall shear stress versus deformation rate at the wall or shear viscosity versus deformation rate determined on the basis of well-defined steady flows.
Rheometers of the prior art include capillary or slit rheometers. They operate by forcing the flowing system through a capillary die or a slit die at a specified volumetric flow rate with a drive mechanism such as a piston or a gear mechanism.
Because shear viscosity of a non-Newtonian flowing system is dependent on the deformation rate of the system, characterization of a non-Newtonian flowing system requires that shear rate and shear stress values be determined at multiple deformation rates to determine the viscosity over the deformation range of interest. Conventional rheometers achieve this by altering the volumetric flow rate into the rheometer. In this way, properties of the flowing system such as the shear rate and shear stress values are altered, and it is possible to determine the viscosity of the particular flowing system of interest over the deformation range of interest. However, altering the flow rate can change the structure and, hence, the rheological properties of the flowing system if a continuous process is used to deliver the flowing system to the rheometer. This distorts the results of the rheological study. Further, generating multiple flow rates requires equipment which is both cumbersome and expensive. Thus, a need exists for a rheometer which can determine the shear viscosity of non-Newtonian flowing systems from a constant flow rate through the rheometer.